The present invention is concerned with the production of microfibrils of cellulose from plant material. Over the past 20 years there has been an increasing interest in extracting and utilising cellulose at a scale below that of the whole cell wall. Native cellulose within plant cell walls exists in the form of microfibrils, which typically have a lateral dimension in the range 2-20 nanometers and a longitudinal dimension of from tens of nanometers to several micrometers, e.g. a length of 100-4000 nm. Cellulose microfibrils are composed entirely of β-1,4-linked glucan chains that form crystalline regions as well as less ordered stretches referred to as amorphous regions.
Cellulose crystallites are very strong, but also very short; too short for many uses e.g. in high performance composites. To fully take advantage of the high strength of cellulose, it becomes important to release microfibrils with as big an aspect ratio as possible and with uncompromised amorphous regions. This is not trivial as β-1,4-linked glucose residues are accessible to enzymes or microorganisms in both hemicellulosic polymers that adhere tightly to cellulose and which shall be removed, and in amorphous regions that should be left intact.
A number of techniques for the liberation of microfibrils from various sources of native cellulose, and for the production of microfibrillated cellulose from same, are known. Typically, wood pulp is used as a source of microfibrillated cellulose. However, in general, methods of obtaining microfibrillated cellulose from wood pulp can involve harsh chemical treatment regimens and/or high energy mechanical treatments, and can have scalability issues.
Cellulose microfibrils can be found in nature from a number of sources other than heavily lignified “woody” tissues. For example, U.S. Pat. No. 5,964,983 discloses a process for the preparation of microfibrillated cellulose from primary wall plant pulp containing cellulose, pectins, hemicelluloses, proteins and mineral materials, which process comprises multi-step chemical treatment involving either acidic or basic hydrolysis of the pulp at 60° C. to 100° C., high mechanical shear treatment followed by high pressure homogenisation. Further, if a decolourized product is required, an additional bleaching step is involved.
State of the art in liberating cellulose microfibrils from herbaceous material is represented by the technology described in WO2006056737. The method comprises controlled fermentation of the more readily digestible parts of the primary plant cell walls by a consortium of microorganisms.
This technology also has some major drawbacks: It is slow; reproducibility is low due to the dynamic nature of the microbial populations; significant amounts of base in the form of bleach are used to sterilize the material and remove matrix polysaccharides that has been rendered more soluble by the fermentation but not digested; and excessive amounts energy are used for mechanical diminution and eventual microfibrillation of the material.
There remains a need to provide a method for producing cellulose microfibrils from plant material which is: an alternative to known methods; is environmentally friendly; involves less process steps and thus is easier to carry out; in which a separate bleaching step is not mandatory; is less energy intensive; in which amorphous regions are less damaged, and, where desired, the cellulose microfibril surface is more efficiently stripped of strongly adhering biopolymers.
It has been proposed in the prior art, most clearly in US2012/0316330, that enzyme-mediated digestion of plant biomass yields both a supernatant of fermentable sugars that may be turned into biofuel, for example, and a solid, cellulosic residue that is useful in various materials, films, composites etc.
However, as disclosed in US2012/0316330, endo-glucanase treatment of a chemically treated pulp also leads to digestion of amorphous regions of cellulose microfibrils as evidenced by a rapid decrease in degree of polymerization as cellulose microfibrils were released. Cellulose microfibrils with partially degraded amorphous regions are weaker than undamaged fibrils. Microfibrils with completely degraded amorphous regions are made up entirely of crystalline regions, and while these are very strong, they are also much shorter and thus have fewer applications in high performance materials. Cellulose microfibril amorphous regions are digested by polysaccharide hydrolases belonging to families where cellulases belong, a non-exhaustive list comprising CAZy families: GH5, GH6, GH7, GH8, GH9, GH12, GH44, GH48.
It is surprising that a method that shall release cellulose microfibrils while keeping them intact, notably the amorphous regions, still has to include enzymes from one or more of these families.